Gary Russell was a child actor, but he's most famous for knowing more about Doctor Who continuity than anyone could ever want to know.
If you wanted to know whether those insect things in The Ark in Space are spelled Wirn or Wirrn, you'd just look it up on the wiki and then forget what you were looking for. Gary dug up the original shooting script, discovered that it's actually spelled Wirrrn, and wrote a whole fucking novel just to make sure everyone else knew that. (Well, to be fair, the novel is also there to make sure the timelines of the Eighth Doctor Adventures novels and the comic strips all fit together properly, because everyone was really worried about that.)
Not all of Gary's stories are designed to let everyone know about how to spell some obscure alien race. For example, he wrote one to make the Sixth Doctor's death make more sense, another on to resolve all the conflicting stories about how the Doctor and the Master knew each other, another to fix the timeline of all the Ice Warrior stories, and of course fucking Zagreus.
The world, at last the world of Doctor Who, would be a much sadder place without Gary Russell. But that doesn't mean you want to actually read any of his books.
If you're British: He's Dick from the 70's Famous Five. If you're not British: Imagine the Hardy Boys go on holiday with Nancy Drew and her lesbian lover whose name I forget, and a dog named Timmy. OK, now make it more mid-century English than you can possibly imagine. No, more English than that. Do you want to punch yourself just for being gay enough to imagine it? If not, get more English. OK, brill! So there's the smart leader boy, the boy who thinks he's funny, the tomboy girl, the girly girl, and the dog. Gary is the one who thinks he's funny.
Gary had many other child acting roles, but his career dried up as a teenager, with his last role being the memorable character of Boy Teenager In Car in Octopussy. Some say it's because he no longer looked like an adorable little English boy, but more likely it's because he kept talking about Doctor Who and in the mid 80's, when nobody in the world wanted to talk about Doctor Who.
Gary then took whatever behind-the-scenes jobs he could get in broadcasting, mostly involving publicity and PR for the BBC. However, they wouldn't assign him do publicity for Doctor Who, based on the flimsy excuse that the show was no longer on the air. So he left to become a games journalist, hoping someone would make a new Doctor Who game, but nobody did.
Doctor Who Edit
By the mid 90's, people started to realize that Doctor Who was no longer on the air and kind of feel nostalgic for it a bit. The world was finally ready for Gary.
Lest you think Gary thinks of nothing but Doctor Who, he also worked on completely unrelated things like BBV's Stranger series (with Colin Baker and Nicola Bryant playing The Stranger and Miss Brown, totally not The Doctor and Peri Brown). But no, seriously, Gary thinks of nothing but Doctor Who.
Besides writing a shit ton of novels and a shit long-ton of audios, Gary edited Doctor Who Magazine, then left that to become executive producer for Big Finish, then became Rusty Davros's official continuity advisor on the telly. But even Rusty didn't want that much continuity advice, so he made Gary go script edit Torchwood and Sarah Jane, and write and direct the animated Doctor Who stories that you probably forgot about but actually weren't that bad.
Eventually, Gary got to script edit the big show itself, for the Tenth Doctor's specials year. There's really only one way up from there. But Wirrrn larvae take too long to incubate, so he couldn't use them to take over Moffat's body in time. So, while waiting for the right time to infest Chris Chibnall, Gary went into hiding in Australia, where he's producing a new sci-fi cartoon that just happens to be named Prisoner Zero.
Gary and his Aussie friends have also been doing animated reconstructions of the missing episodes—kind of like Ian Levine, except that they get contracts and sort out the rights first, instead of doing the work on spec and then threatening to hold their breath until someone gives them money.